1. Field of the Invention
The present invention relates to an image reading apparatus for forming an image of a light reflected from a document surface as a document image on a light-receiving surface of an imaging device, and an image forming apparatus, e.g. a copying machine, a facsimile machine, and a complex machine having functions of those, which is provided with the image reading apparatus.
2. Description of the Related Art
In recent years, an image forming apparatus such as a copying machine and a printer has been generally used which has an image forming section operable to form a toner image on a surface of a photoconductive drum on which an electrostatic latent image is formed and then transfer the toner image onto a recording sheet. In such image forming apparatus, there is provided an image reading apparatus which uses an imaging lens to form an image of a reflected light, which is obtained by irradiating a light to a document surface from a light source (exposure light source) constituting an illuminating portion, as a document image onto a light-receiving surface of an imaging device (photoelectric conversion element) as a line sensor such as a CCD (Charge Coupled Device).
In the conventional image reading apparatus so configured as described above, a rod-like light source such as a xenon lamp is generally used as a light source for irradiating a light to a document surface. According to such illuminating portion using a xenon lamp, which is a rod-like light source, an illuminance distribution in sub-scanning direction which is a relative moving direction with respect to the light source and the document surface at a time of reading a document, (a light distribution over a cross section in sub-scanning direction) has a flat peak characteristic as shown in FIG. 9, so that an illumination efficiency is bad. Accordingly, there has been a problem that it would be necessary to improve an ability to converge a light in sub-scanning direction, and it would be also necessary to prevent image deterioration caused by lowering in light intensity at end portions in the main scanning direction, which is a direction perpendicular to the sub-scanning direction, of the imaging device due to an effect of deformation caused by the imaging lens.
Therefore, it has been proposed to align a plurality light-emitting diodes in the main scanning direction, and on the other hand arrange the light-emitting diodes so as to be not densely at a central portion in the line direction but densely at end portions, and to provide a collective lens between the light source and the document surface (for example, Japanese Patent Unexamined Publication No. H2-284571, referred to as a patent document 1 hereinafter). In the illuminating portion so configured as described above, since the illuminance distribution in the sub-scanning direction of the light-emitting diodes has a sharp-pointed peak characteristic as shown in FIG. 10, and the collective lens is provided between the light source and the document surface, excellent illumination efficiency can be achieved. Further, since the light-emitting diodes are arranged densely in peripheral portions, the light intensity at end portions of the light source increases. Consequently, it causes lowering of the light intensity at end portions in the main scanning direction of the imaging device to be less likely to occur, so that the image deterioration can be prevented.
However, even though good illumination efficiency can be achieved, and image deterioration at end portions in the main scanning direction of the imaging device can be prevented according to the proposed image forming apparatus, it causes problems that increasing the number of light-emitting diodes at the end portions causes costs to rise, and manufacturing of the light source becomes complicated since it would be necessary to arrange the pitch of the light-emitting diodes at the central portion be different from that at the end portions.